Ceramic pin heating element with integrated connector contacts and method for making same

ABSTRACT

A ceramic pin heating element having exterior heating conductors and integrated electrical connection surfaces, and method for making same. A first body is injection molded from a first injection-moldable ceramic composite compound having a first electrical resistance, and a second body is injection-molded from a second injection-moldable ceramic composite compound having a second electrical resistance about the first body so as to form a compound body. The compound body is then sintered. The resulting ceramic pin heating element is directly formed without additional steps for protecting (insulating) the element so as to prevent the exterior, complementary heating conductors from contacting the housing and/or terminal bolts. The danger of breakage of the pins at the mounting location and/or at the seal seat through variations in the thickness of contact and/or insulation coatings is minimized.

FIELD OF THE INVENTION

The present invention relates to ceramic pin heating elements, and in particular, to a ceramic pin heating element for an internal combustion engine, the pin heating element having integrated connector contacts.

BACKGROUND INFORMATION

Connection of ceramic pin heating elements having exterior heating element shafts in a metal housing with simultaneous sealing from the combustion chamber is complex. The installation position causes contact between the heating element shafts having different polarity and the grounded housing. Therefore, to avoid short circuits, one of the two heating element shafts must be electrically insulated from the housing in the area of the seal seat. Also, the other heating element shaft, having opposite polarity, must be electrically insulated from the connector bolt. The application of electrical insulation layers is currently implemented through separate vitrification steps or through application, for example by printing, and burning-in of electrically insulating layers. To avoid one-sided mechanical stresses of the installed pin heating element due to the applied insulation layers, the corresponding open connector shaft is also provided with a layer of the same thickness but with electrical conductive properties, since there might otherwise be a danger of breakage of the ceramic pin heating element in the area of the seal seat. Another possibility is for the ceramic pin heating element to be fully insulated from the housing through application of a coating. As with a potential-free A-probe, contact is made through a divided terminal bolt.

SUMMARY OF THE INVENTION

The present invention provides a method for making a ceramic pin heating element having exterior conductors and integrated electrical connection surfaces. The method comprises injection molding a first body from a first injection-moldable ceramic composite compound having a first electrical resistance; injection molding a second injection-moldable ceramic composite compound having a second electrical resistance about the first body so as to form a compound body; and sintering the compound body.

The present invention also provides a ceramic pin heating element comprising a first body injection molded from a first injection-moldable ceramic composite compound, the first body having a first electrical resistance, and a second body injection molded from a second injection-moldable ceramic composite compound, the second body having a second electrical resistance. The first body and second body form a compound body, the compound body being sintered after injection molding of the first body and the second body.

A ceramic pin heating element according to an embodiment of the present invention has exterior heating conductors based on Si₃N₄/MSi₂, where M is at least one of Mo, Nb, Ti and W, and has integrated electrical connection surfaces using multiple-component injection-molding technology.

An advantage of the method and heating element of the present invention is direct shaping of the pin heating element without additional steps for protecting (insulating) the element so as to prevent the exterior, complementary heating conductors from contacting the housing and/or terminal bolts. Another advantage according to the present invention is minimization of the danger of breakage of the pins at the mounting location and/or at the seal seat through variations in the thickness of contact and/or insulation coatings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a schematic cross-sectional view of a ceramic pin heating element according to the present invention.

FIG. 1B shows a cross-section through line B—B of FIG. 1A.

FIG. 1C shows a cross-section through line C—C of FIG. 1A.

FIG. 1D shows a cross-section through line D—D of FIG. 1A.

FIG. 2 shows a flow chart of a method according to the present invention.

DETAILED DESCRIPTION

Referring to FIG. 1A, an exemplary ceramic pin heating element of the present invention includes a heating conductor 2, a housing 12, an insulator body 22 and a connector bolt 32. Heating conductor 2 has a negative pole 3, a positive pole 6 and a tip 9, which is heated during operation of the heating element. Negative pole 3 has a lower section 4 disposed in housing 12, and a generally semi-cylindrical upper section 5 projecting from housing 12. Positive pole 6 has a lower section 7 disposed in housing 12, and a generally semi-cylindrical upper section 8 projecting from housing 12. Housing 12 forms a generally cylindrical, annular casing around lower sections 4 and 7 of negative pole 3 and positive pole 6, respectively, and around portions of insulator 22. Insulator 22 has a generally cylindrical stem section 24 encased by negative pole 3 and positive pole 6, a curved section 26 disposed between housing 12 and a portion of lower section 4 of negative pole 3, and a semi-disc shaped section 28 disposed between connection bolt 32 and lower section 4 of negative pole 3. As embodied herein, connection bolt 32 is electrically connected to positive pole 6 and housing 12 is electrically connected to negative pole 3.

Heating conductor 2 is made of an injection-moldable ceramic composite compound having a relatively high conductivity. Insulator 22 is made of an injection-moldable ceramic composite compound having a relatively low conductivity. Thus, one of the injection-moldable ceramic composite compounds having a relatively high conductivity is substantially conductive, and the other one of the injection-moldable ceramic composite compounds having a relatively low conductivity is substantially insulative. Housing 12 is preferably made of free-cutting steel. Alternatively, housing 12 may be made of other suitable metals or materials. Connector bolt 8 is also preferably also made of free-cutting steel, but may also be made of other suitable metals or materials. FIGS. 1B-1D show cross-sectional views through the heating element of FIG. 1A at various levels.

Referring now to FIG. 2, an exemplary method according to the present invention includes injection molding a first body from a first injection-moldable ceramic composite compound having a first electrical resistance, as shown in block 102. As shown in block 104, the next step of an exemplary method of the present invention is injection molding a second injection-moldable ceramic composite compound having a second electrical resistance about the first body so as to form a compound body. The first body may form heating element 2 or, alternatively, insulator body 22 of the ceramic pin heating element, while the second injection-moldable ceramic composite makes up, respectively, insulator body 22 or heating element 2. Sintering the compound body is then performed, as shown in block 106.

The ceramic pin heater is produced using suitable organic process aids. As embodied herein, suitable process aids include grafted polypropylene, such as POLYBOND 1001 made by Uniroyal Chemical company, in combination with cyclododecane and/or cyclododecanol. Other suitable organic process aids include combinations of polyolefin waxes, such as HOSTAMONT TPEK 583 made by the firm Ticone GmbH, or of polyoxymethylene, such as CATAMOLD made by the firm BASF AG. The suitable organic process aid(s) are added to composites Si₃N₄/MSi₂ (where M is at least one of Mo, Nb, W and Ti), the composites having differing specific electrical resistances following sintering. The composites are then formed into molded bodies through injection molding. The sintering is then performed, preferably as a two-step sintering process including a presintering and a primary sintering step.

A method in accordance with the present invention uses a preconditioned SI₃N₄ powder with appropriate sintering additives such as Al₂O₃, Y₂O₃, and the like and an admixture of MSi₂ (M: Mo, Nb, W, Ti) in differing proportions. The admixture of MSi₂ is such that a component A, which is highly insulating following the sintering fire, and a very highly conductive component B, are created.

Injection-moldable compounds AS and BS are prepared from the preconditioned ceramic powder mixtures A and B with a suitable organic binder system which according to an exemplary embodiment of the present invention is composed of grafted polypropylene in combination with cyclododecane and/or cyclododecanol.

An insulation body made of compound AS is formed by injection molding. A conductive compound BS is injection molded around the insulation body AS, for example in the two-component injection molding procedure.

Debinding and presintering is performed under an inert gas at a pressure of 1 bar and a temperature of up to 900° C. A primary sintering process follows the forming process after debinding and presintering. Primary sintering occurs under a defined N₂ partial pressure with the N₂ partial pressure being not higher than 10 bar and the total sintering pressure being elevated through addition of an inert gas, such as, for example, Ar, to values up to 100 bar, the sintering gas being at a temperature between 1000° C. and the sintering temperature which is no higher than 1900° C.

Alternatively, primary sintering is performed under a defined N₂ partial pressure with the N₂ partial pressure varied with the temperature so that the partial pressure is within a range which is bounded by the following relationships and the total sintering pressure being elevated through addition of an inert gas, such as, for example, Ar, to values up to 100 bar:

Upper limit: log p(N ₂)=7.1566·In(T)−52.719

Lower limit: log p(N ₂)=9.8279·In(T)−73.988.

Where T is the sintering temperature in ° C. and p(N₂) is the partial pressure of N₂ in bar. The sintering temperature is no higher than 1900° C.

The electrical contact surfaces may also be provided with a thin metal coating such as one based on Ni or the like.

In an exemplary embodiment of the present invention a composite powder A of 54 wt % Si₃N₄, 2.58 wt % Al₂O₃, 3.42 wt % Y₂O₃, and 40 wt % MoSi₂ and a composite powder B of 36 wt % Si₃N₄, 1.72 wt % Al₂O₃, 2.28 wt % Y₂O₃, and 60 wt % MoSi₂ are produced. The mean particle size of the Si₃N₄ used is 0.7 μm, and that of the MoSi₂ used is 1.8 μm. From these composite powders, injection-moldable compounds AS and BS are produced. Injection-moldable powder compounds represent highly filled dispersions.

A binder system suitable for powder injection molding preferably meets the following requirements: (1) dispersion effect for avoidance of clumping of powder; (2) good flow quality of the compounds during injection molding; (3) adequate bonding of a second compound injected over a blank; (4) low formation of pyrolysis carbon during thermal debinding in inert gas atmosphere and in air, since carbon negatively influences the properties of the sintered molded body; and (5) rapid debinding without formation of defects.

The combination of grafted polypropylenes and cyclododecane and/or cyclododecanol in accordance with a method of the present invention, for example, represents a binder system of this type. The polar compounds grafted onto the polypropylene chain such as acrylic acid or maleic acid anhydride attach to the surfaces of the powder. The polypropylene POLYBOND 1001 used in the present exemplary embodiment is a homo-polypropylene grafted with 6% acrylic acid of the company Uniroyal Chemical.

For production of the compound BS from the conductive ceramic, 82 wt % of preconditioned powder mixture B is kneaded with 12 wt % POLYBOND 1001 and 6 wt % cyclododecane under protective gas at 180° C. and is granulated through cooling with the kneader running.

In this way, a compound AS is produced from the insulating ceramic powder A, the filler content of which is matched to that of the compound BS such that test bodies injection molded and debinded from both compounds under the same sintering conditions have the same sintering shrinkage.

First the conductive body is formed with the compound BS through two-component injection molding. After withdrawal or change of the pusher which forms the recesses for the insulation areas in the injection molding tool, the voids formed are injected with the compound AS. In this process, a bond develops between the insulating body and the conductive path.

After thermal debinding and sintering according to sintering conditions 2, the AS component has a specific resistance of 10⁷ Ωcm and the BS component has a specific resistance of 6*10⁻³ Ωcm. 

What is claimed is:
 1. A method for making a ceramic pin heating element having exterior heating conductors and integrated electrical connection surfaces, the method comprising the steps of: injection molding a first body from a first injection-moldable ceramic composite compound having a first electrical resistance; injection molding a second injection-moldable ceramic composite compound having a second electrical resistance about the first body so as to form a compound body; and sintering the compound body; wherein the first electrical resistance and the second electrical resistance differ.
 2. The method according to claim 1 wherein the first injection-moldable ceramic composite compound is substantially conductive and the second injection-moldable ceramic composite compound is substantially insulative.
 3. The method according to claim 1 wherein the first injection-moldable ceramic composite compound is substantially insulative and the second injection-moldable ceramic composite compound is substantially conductive.
 4. The method according to claim 1 wherein the first injection-moldable ceramic composite compound includes trisilicon tetranitride, a first metal silicide and a first binder system, and wherein the second injection-moldable ceramic composite compound includes trisilicon tetranitride, a second metal silicide and a second binder system.
 5. The method according to claim 4 wherein at least one of the first and second binder systems includes at least one grafted polypropylene and at least one of cyclododecane and dyclodecanol.
 6. The method according to claim 5 wherein the at least one grafted polypropylene includes a homopolypropylene grafted with 6% acrylic acid.
 7. The method according to claim 4 wherein the first metal silicide is of the form (M1)Si₂, (M1) being a metal selected from Mo, Nb, W, and Ti, and wherein the second metal silicide is of the form (M2)Si₂, (M2) being a metal selected from Mo, Nb, W, and Ti.
 8. The method according to claim 4 wherein the first and second metal silicides are the same metal silicide.
 9. The method according to claim 4 wherein the first injection-moldable composite compound includes a first sintering additive and the second injection-moldable composite compound includes a second sintering additive, at least one of the first and second sintering additives including at least one of Al₂O₃ and Y₂O₃.
 10. The method according to claim 1 further comprising the step of presintering the compound body.
 11. The method according to claim 10 wherein the presintering takes place at atmospheric pressure under an inert gas at a maximum temperature of 900° C.
 12. The method according to claim 1 wherein the sintering takes place under a nitrogen partial pressure, a maximum value of the nitrogen partial pressure being 10 bar when a temperature of the sintering is between 1000° C. and 1900° C.
 13. The method according to claim 1 wherein a temperature of the sintering is a maximum of 1900° C.
 14. The method according to claim 1 further comprising the step of debinding the compound body.
 15. The method according to claim 1 further comprising coating at least a portion of the electrical connection surfaces with a metal coating.
 16. The method according to claim 1, wherein: the first injection-moldable ceramic composite compound includes trisilicon tetranitride, a first metal silicide and a first binder system, and the second injection-moldable ceramic composite compound includes trisilicon tetranitride, a second metal silicide and a second binder system; at least one of the first and second binder systems includes at least one grafted polypropylene and at least one of cyclododecane and dyclodecanol; and the first metal silicide is of the form (M1)Si₂, (M1) being a metal selected from Mo, Nb, W, and Ti, and the second metal silicide is of the form (M2)Si₂, (M2) being a metal selected from Mo, Nb, W, and Ti.
 17. The method according to claim 16, wherein the at least one grafted polypropylene includes a homopolypropylene grafted with 6% acrylic acid.
 18. The method according to claim 1, wherein: the first injection-moldable ceramic composite compound includes trisilicon tetranitride, a first metal silicide and a first binder system, and the second injection-moldable ceramic composite compound includes trisilicon tetranitride, a second metal silicide and a second binder system; the first and second metal silicides are the same metal silicide; the first injection-moldable composite compound includes a first sintering additive and the second injection-moldable composite compound includes a second sintering additive, at least one of the first and second sintering additives including at least one of Al₂O₃ and Y₂O₃. 